Light-emitting diode device structure with SixNy layer

ABSTRACT

A light-emitting diode (LED) structure fabricated with a Si x N y  layer responsible for providing increased light extraction out of a surface of the LED is provided. Such LED structures fabricated with a Si x N y  layer may have increased luminous efficiency when compared to conventional LED structures fabricated without a Si x N y  layer. Methods for creating such LED structures are also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of co-pending U.S. patentapplication Ser. No. 12/120,768, filed May 15, 2008, which is hereinincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present invention generally relate to the field oflight-emitting diode (LED) technology and, more particularly, to alight-emitting diode (LED) structure with increased light extraction.

2. Description of the Related Art

Luminous efficiency of LEDs can be defined as the total apparent powerof a light source to its actual total input power (luminous flux dividedby input power). Having units of lumens per watt, luminous efficiencymeasures the fraction of power which is useful for lighting. As a typeof light source, light-emitting diodes (LEDs) have been designed anddeveloped over the past few decades to make improvements in luminousefficiency and increase the number of possible applications for thesesolid state devices.

Beginning with a conventional LED structure whose cross-section is shownin FIG. 1, one can see why the luminous efficiency of these devices isrelatively poor. A conventional LED 100 is formed on a substrate 104such as sapphire, silicon carbide, silicon, germanium, ZnO or galliumarsenide depending on the composition of the LED layers to be deposited.An n-doped layer 102 is disposed above the substrate 104, and this layer102 may comprise n-doped GaN. GaN may be grown on a sapphire substratefor emitting green to ultraviolet (UV) wavelengths of light. A multiplequantum well (MQW) active layer 103 is deposited above the n-doped layer112, and this is where photon generation occurs when the diode isproperly biased. A p-doped layer 106 is grown above the active layer 103in FIG. 1. After portions of the p-doped layer 106 and the active layer103 are removed to expose a portion of the n-doped layer 102, electrodes108 and 110 may be formed on the p-doped and n-doped layers,respectively, for forward biasing the LED.

To improve upon some of the design limitations for luminous efficiencyof conventional LEDs, the vertical light-emitting diode (VLED) structurewas created. The VLED earned its name because the current flowsvertically from p-electrode to n-electrode, and a typical VLED 200 isshown in FIG. 2. To create the VLED 200, an n-doped layer 102 isdeposited on a substrate (not shown), and this may comprise any suitablesemiconductor material for emitting the desired wavelength of light,such as n-GaN or a combination of undoped GaN and n-GaN. A multiplequantum well (MQW) active layer 103 from which the photons are emittedis grown above the n-doped layer 102. A p-doped layer 106 is depositedabove the active layer 103 in FIG. 2. A metal layer 202 may be depositedabove the p-doped layer 106 for electrical conduction and heatdissipation away from the VLED.

Unwanted dislocations 112 may form in an LED during the growing of oneor more of the layers that make up the LED. In conventional LEDs,current flows along the surface very far from the interface of thesubstrate 104 and the n-doped layer 102 so the effects of dislocationson current are not obvious. Unwanted dislocations may also occur in aVLED, and because the dislocations in a VLED may run in the direction ofthe current, reductions in the dislocation density may have a morenoticeable effect on decreasing the leakage current. Leakage current, asdefined herein, generally refers to the current measured when −5V ofreversed bias is applied to the LED electrodes.

Accordingly, what is needed is a light-emitting solid state device withreduced dislocation density and increased luminous efficiency.

SUMMARY OF THE INVENTION

One embodiment of the present invention provides a method of increasinglight extraction from a light-emitting diode (LED) device. The methodgenerally includes depositing a first n-doped layer above a carriersubstrate; depositing a Si_(x)N_(y) mask above the first n-doped layer,wherein the Si_(x)N_(y) mask has openings exposing portions of the firstn-doped layer; depositing a second n-doped layer above the Si_(x)N_(y)mask such that the second n-doped layer is also deposited in theopenings; and depositing a p-doped layer above the active layer, suchthat the Si_(x)N_(y) mask reduces dislocations in at least one of thesecond n-doped layer, the active layer, and the p-doped layer.

Another embodiment of the present invention provides an LED device. TheLED device generally includes a carrier substrate; a first n-doped layerdisposed above the carrier substrate; a Si_(x)N_(y) mask disposed abovethe first n-doped layer, wherein the Si_(x)N_(y) mask has openingsexposing portions of the first n-doped layer; a second n-doped layerdisposed above the Si_(x)N_(y) mask such that the second n-doped layeris also disposed in the openings; an active layer for emitting lightdisposed above the second n-doped layer; and a p-doped layer disposedabove the active layer, wherein a first dislocation density above theSi_(x)N_(y) mask is lower than a second dislocation density below theSi_(x)N_(y) mask.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 is a cross-sectional schematic representation of a prior artlight-emitting diode (LED) structure;

FIG. 2 is a cross-sectional schematic representation of a prior artvertical light-emitting diode (VLED) structure;

FIGS. 3A-3F illustrate fabrication of an LED with a Si_(x)N_(y) layeraccording to an embodiment of the invention;

FIGS. 4A and 4B are cross-sectional schematic representations of LEDstructures fabricated with a Si_(x)N_(y) layer with different amounts ofetching according to embodiments of the invention;

FIGS. 5A and 5B compare surface views of n-doped layers of LEDstructures fabricated without and with a Si_(x)N_(y) layer,respectively;

FIGS. 6A and 6B illustrate fabrication of an LED with a Si_(x)N_(y)layer according to embodiments of the invention; and

FIG. 6C is a cross-sectional schematic representation of an LEDstructure fabricated with a Si_(x)N_(y) layer according to an embodimentof the invention.

DETAILED DESCRIPTION

Embodiments of the present invention provide a light-emitting diode(LED) structure fabricated with a Si_(x)N_(y) layer responsible forproviding increased light extraction out of a surface of the LED.Embodiments of the present invention also provide for a method ofcreating such an LED.

An Exemplary Led Device Having a Si_(x)N_(y) Layer

The depositing of a Si_(x)N_(y) layer may reduce the dislocation densityin the layers of an LED and lead to increased light extraction. FIGS.3A-3F illustrate various steps in the fabrication of an LED according toan embodiment of the invention.

In FIG. 3A, a first n-doped layer 304 may be deposited on a carriersubstrate 302. The first n-doped layer 304 may comprise any suitablesemiconductor material for LED functionality, such as n-doped GaN(n-GaN) or a combination of undoped GaN and n-GaN. Any suitable materialsuch as sapphire, silicon carbide (SiC), silicon, germanium, zinc oxide(ZnO), or gallium arsenide (GaAs) may be used as the carrier substrate302.

The depositing of the first n-doped layer 304 may be performed using anysuitable thin film deposition techniques, such as electrochemicaldeposition, electroless chemical deposition, chemical vapor deposition(CVD), metal organic vapor phase epitaxy, metal organic CVD (MOCVD),plasma enhanced CVD (PECVD), atomic layer deposition (ALD), physicalvapor deposition (PVD), evaporation, plasma spray, or a combination ofthese techniques. Dislocations 306 may form in the first n-doped layer304. The thickness of the first n-doped layer 304 may be any suitablethickness, such as in a range from 0.1 to 10 microns. When MOCVD is usedto deposit the first n-doped layer, a first n-doped layer thickness ofat least 0.5 microns may be suitable for increased light extraction.

In FIG. 3B, a Si_(x)N_(y) mask 308 may be deposited above the firstn-doped layer 304. The Si_(x)N_(y) mask 308 may partially cover thefirst n-doped layer leaving a connecting area for the first n-dopedlayer 304 and a second n-doped layer 310 that may be deposited later(see FIG. 3C). The coverage of the Si_(x)N_(y) mask may not becontinuous and may leave an area of the first n-doped layer exposed forsubsequent growth of another n-doped layer.

The Si_(x)N_(y) mask 308 may be deposited by any suitable technique suchas depositing individual “islands” through sputtering or by depositing acontinuous Si_(x)N_(y) layer and then removing parts of the continuouslayer to form individual islands. Using MOCVD, the Si_(x)N_(y) mask 308may be grown using silane (SiH₄), disilane (Si₂H₆), or any derivative ofsilane (Si_(n)H_(2n+2)) combined with NH₃ (or any material containingnitrogen that can decompose at growth temperatures and emits nitrogen).

The Si_(x)N_(y) mask may be formed using in situ deposition where theSi_(x)N_(y) mask is created in the process chamber and used as a mask inthe position where it was created. The Si_(x)N_(y) mask 308 may bedeposited with any suitable growth temperature that is higher than thegrowth temperature of GaN (higher by 5 to 100° C.) especially withMOCVD. When the growth temperature of the Si_(x)N_(y) mask is higherthan the growth temperature of GaN, the leakage current of the LED maybe reduced. The shape of the Si_(x)N_(y) islands deposited above thefirst n-doped layer 304 may be any suitable shape created by any of theabove-mentioned processes of depositing the Si_(x)N_(y) mask such asrectangular, trapezoidal, or pyramidal.

The Si_(x)N_(y) mask 308 may be deposited with any suitable thickness aslong as the mask does not cover the whole first n-doped layer. Increasedcoverage of the Si_(x)N_(y) mask islands may indicate that a thickerlayer may be required to recover a continuous n-doped layer after theSi_(x)N_(y) mask. In order to create an LED that is practical for massproduction, the coverage of the Si_(x)N_(y) mask is chosen so asubsequent n-doped layer recovers after a suitable growth thickness suchas 4 to 8 microns. Using MOCVD, the growth pressure of the Si_(x)N_(y)mask may not be as important as the growth temperature. As long as thegrowth temperature of the Si_(x)N_(y) mask is higher than the n-dopedlayer's growth temperature (e.g., 5-100° C. higher), the subsequentn-doped layer (a second n-doped layer described below) may recover withmuch less leakage current in the resulting LED.

In FIG. 3C, a second n-doped layer 310 may be deposited above theSi_(x)N_(y) mask and may be formed using any of the above-describeddeposition techniques. The second n-doped layer 310 may containdislocations 306 in areas that were not covered by the Si_(x)N_(y) mask308. Some dislocations may have been prevented from developing above thelayer with the Si_(x)N_(y) mask 308 in the second n-doped layer 310resulting in decreased dislocation density in the second n-doped layer310 and thus, lower leakage current in the resulting LED structure.

In FIG. 3D, an active layer 314 for emitting light may be depositedabove the second n-doped layer, and a p-doped layer 312 may be depositedabove the active layer. Both layers 312, 314 may be formed using any ofthe above-described deposition techniques. In FIG. 3E, one or more metallayers 316 may be deposited above the p-doped layer 312 rather thanattached with wafer bonding or gluing. The one or more metal layers 316may be deposited using any suitable thin film deposition technique, suchas physical vapor deposition (PVD), chemical vapor deposition (CVD),plasma enhanced CVD (PECVD), evaporation, ion beam deposition,electrochemical deposition, electroless chemical deposition, plasmaspray, or ink jet deposition. The metal layer may comprise any suitablematerial for electrical and thermal conduction, such as chromium (Cr),platinum (Pt), nickel (Ni), copper (Cu), Cu on a barrier metal material(e.g., titanium nitride, tungsten, tungsten nitride, tantalum nitride),molybdenum (Mo), tungsten (W) or a metal alloy. One or more of the metallayers may be formed by electrochemical plating or electroless chemicalplating. One or more metal layers may be deposited on a seed metallayer. The seed metal layer may be grown via electroless plating andassist in the growth of a single metal layer or of multiple metal layersvia electroplating.

In FIG. 3F, the carrier substrate 302 may be removed. The carriersubstrate removal may be done using laser, etching, grinding/lapping,chemical mechanical polishing (CMP), wet etching, or any other suitabletechnique. After the carrier substrate is removed, the exposed surface317 of the first n-doped layer may be roughened in an effort to increasethe light extraction according to Snell's law. The roughening may occurthrough any suitable technique such as through a photoelectrochemicaloxidation process as described below, by wet etching, or by dry etching.An electrode 318 for external connection may be deposited on the surface317 after roughening.

Roughening may occur using any suitable process, such as the processdescribed in U.S. Pat. No. 7,186,580 to Tran, issued Mar. 6, 2007 andentitled “LIGHT EMITTING DIODES (LEDS) WITH IMPROVED LIGHT EXTRACTION BYROUGHENING.” One embodiment teaches the photoelectrochemical (PEC)oxidation and etching of the n-doped layer. PEC oxidation and etchingmay be performed in a system with an aqueous solution, an illuminationsystem, and an electrically biased system. The aqueous solution may be acombination of oxidizing agent and either acid or alkaline solutions.The oxidizing agent may be any suitable agent such as one or thecombination of H₂O₂ and K₂S₂O₈, among others. The acid solution may beany suitable solution, such as one or more of H₂SO₄, HF, HCl, H₃PO₄,HNO₃, and CH₃COOH. The alkaline solution may be any suitable solution,such as one or the mixture of KOH, NaOH, and NH₄OH. The illumination maybe performed through any suitable method, such as by an Hg or Xe arclamp system with wavelength ranging from the visible to the ultravioletspectrum. The illumination may be exposed on the n-type III-nitridesemiconductors with an intensity less than 200 mW/cm². An electricalbias may be applied to the conductive substrate, and the voltage may becontrolled between −10 and +10 V. The oxidation-dominant, theetching-dominant, or the combined reactions may be controlled in anysuitable method, such as being controlled to optimize the roughness ofthe n-GaN surface by varying the constitution of the aqueous solution,the electrical bias, and/or the illumination intensity. The non-orderedtextured morphology also may be revealed after the roughening process.

FIG. 4A illustrates a cross-sectional schematic representation of alight-emitting diode (LED) structure 400 according to an embodiment ofthe invention. After the above-described process, the Si_(x)N_(y) mask308 may partially cover the first n-doped layer 304, and a surface 402of the first n-doped layer 304 may have been roughened as describedabove.

The LED 410 illustrated in FIG. 4B results from the roughening processdescribed in FIG. 4A continuing until the Si_(x)N_(y) mask 308 has beenremoved. Part of the second n-doped layer 310 may be removed, as well.An electrode 318 may be disposed on the roughened surface 412 asillustrated in FIG. 4B. The roughened surface 412 of the LED may mostlikely result in greater light extraction than if the Si_(x)N_(y) maskwas not present initially.

An Exemplary Surface Roughness Due to the Si_(x)N_(y) Layer

The presence of Si_(x)N_(y) mask “islands” partially covering the firstn-doped layer 304 may reduce the dislocation density in one or more ofthe second n-doped layer 310, the active layer 314 and the p-doped layer312. In certain LEDs, current flows along the direction of thedislocations and so more dislocations can mean more leakage current inthe LED. The reduction of the dislocation density may most likely reducethe leakage current in the LED.

Additionally, the presence of the Si_(x)N_(y) mask may provide forincreased light extraction from the surface 402, when roughened. Thepresence of the Si_(x)N_(y) mask “islands” may alter the rougheningprocess and create a more even roughening with greater surface area onthe exposed surface of the first n-doped layer or the second n-dopedlayer and increased light extraction from the LED. An LED with aSi_(x)N_(y) mask as described above may provide for more even rougheningon the first or the second n-doped layer depending on the depth ofetching and, hence, more light extraction than a conventional LEDwithout the Si_(x)N_(y) mask.

The presence of the Si_(x)N_(y) mask partially covering the firstn-doped layer, once exposed by removing the first n-doped layer, maylead to differently shaped surface structures on the second n-dopedlayer (e.g., higher density of the pyramid structures at the surfaceresulting in a greater effective surface area for photons to escape)when compared to other LED structures fabricated without the Si_(x)N_(y)mask. Resulting surface structures created on the second n-doped layermay be high density smaller pyramidal or conical structures. Thesestructures may increase the light extraction of the LED. Therefore, theSi_(x)N_(y) mask may not only decrease the leakage current, but may alsoincrease the light extraction of the LED.

FIGS. 5A and 5B illustrate exemplary surfaces that may occur afterroughening. The light-emitting surface 510 of the second n-doped layerin an LED fabricated with a Si_(x)N_(y) mask shown in FIG. 5B has moreeven roughening with higher density of pyramidal structures resulting ingreater surface area than the light-emitting surface 500 of the n-dopedlayer in an LED fabricated without the Si_(x)N_(y) mask shown in FIG.5A. Therefore, the LED fabricated with a Si_(x)N_(y) mask and secondn-doped layer exposed as shown in FIG. 5B may most likely have morelight extraction according to Snell's Law and, thus, greater luminousefficiency when compared to LEDs fabricated without the Si_(x)N_(y)mask.

Another Exemplary Led Device Having a Si_(x)N_(y) Layer

As described above, unwanted dislocations may form in an LED during thegrowing of one or more of the layers that make up the LED. Thedepositing of a Si_(x)N_(y) layer may reduce the dislocation density inthe layers of an LED and lead to increased light extraction.

FIG. 6C illustrates a cross-sectional schematic representation of an LEDstructure 600 according to an embodiment of the invention. The LEDstructure 600 may be fabricated as described above with reference toFIGS. 3A-D. Thereafter, portions of the p-doped layer 312 and the activelayer 314 may be removed to expose a portion of the second n-doped layer310, as illustrated in FIG. 6A. As shown in FIG. 6B, a p-electrode 608may be deposited on the p-doped layer 312. For some embodiments, asillustrated in FIG. 6C, an n-electrode 610 may be deposited on thesecond n-doped layer 310, for forward biasing the LED.

The presence of the Si_(x)N_(y) mask 308 partially covering the firstn-doped layer 304 may reduce the dislocation density in at least one ofthe second n-doped layer 310, the active layer 314, and the p-dopedlayer 312 (La, in layers above the Si_(x)N_(y) mask 308). For example,some dislocations may have been prevented from developing above thelayer with the Si_(x)N_(y) mask 308 in at least the second n-doped layer310, as illustrated in FIG. 3C. As a result, the reduction of thedislocation density may most likely reduce the leakage current in theLED structure 600, thereby leading to increased light extraction from anupper surface of the p-doped layer 312.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

The invention claimed is:
 1. A method of increasing light extractionfrom a light-emitting diode (LED) device comprising; forming a firstn-doped layer on a carrier substrate; forming a Si_(x)N_(y) mask on thefirst n-doped layer; forming a second n-doped layer on the Si_(x)N_(y)mask; forming an active layer configured to emit light on the secondn-doped layer; forming a p-doped layer on the active layer; removing thecarrier substrate; and forming a plurality of structures on the secondn-doped layer by removing the first n-doped and the Si_(x)N_(y) mask andthen roughening a surface of the second n-doped layer.
 2. The method ofclaim 1 wherein the forming the plurality of structures step comprisesetching.
 3. The method of claim 1 wherein the forming the plurality ofstructures step comprises photoelectrochemical (PEC) oxidation andetching.
 4. The method of claim 1 wherein the Si_(x)N_(y) mask isconfigured to reduce dislocations in the second n-doped layer during theforming the plurality of structures step.
 5. The method of claim 1wherein the Si_(x)N_(y) mask is configured to reduce dislocations in theactive layer during the forming the plurality of structures step.
 6. Themethod of claim 1 wherein the Si_(x)N_(y) mask is configured to reducedislocations in the p-doped layer during the forming the plurality ofstructures step.
 7. The method of claim 1 wherein the Si_(x)N_(y) maskis configured to increase a density of the structures relative toforming the structures without the Si_(x)N_(y) mask.
 8. The method ofclaim 1 wherein the Si_(x)N_(y) mask is configured to increase a surfacearea of the structures relative to forming the structures without theSi_(x)N_(y) mask.
 9. The method of claim 1 wherein the forming theplurality of structures step comprises etching to remove portions of thesecond n-doped layer.
 10. The method of claim 1 further comprisingforming a p-electrode on the p-doped layer.
 11. The method of claim 1,further comprising: forming an n-electrode on the second n-doped layer.12. The method of claim 1 wherein the Si_(x)N_(y) mask is configured toreduce current leakage in the LED device.
 13. A light-emitting diode(LED) device comprising: a carrier substrate; a first n-doped layer onthe carrier substrate having a first dislocation density; a Si_(x)N_(y)mask on the first n-doped layer; a second n-doped layer on theSi_(x)N_(y) having a plurality of structures configured to increaselight extraction and a second dislocation density lower than the firstdislocation density, the Si_(x)N_(y) mask located between the firstn-doped layer and the second n-doped layer and configured to lower thesecond dislocation density and form the structures with an increaseddensity and surface area; an active layer configured to emit light onthe second n-doped layer; and a p-doped layer on the active layer. 14.The LED device of claim 13 wherein the Si_(x)N_(y) mask is configured toreduce dislocations in the active layer.
 15. The LED device of claim 13wherein the Si_(x)N_(y) mask is configured to reduce dislocations in thep-doped layer.
 16. The LED device of claim 13 further comprising: ap-electrode on the p-doped layer.
 17. The LED device of claim 13 furthercomprising: an n-electrode on the second n-doped layer.
 18. The LEDdevice of claim 13 further comprising a metal layer on the p-dopedlayer.